We have to swim in school...?

so I just started swimming for P.E., and I suck at it, I don't know any stokes except free style. Also I keep getting foot cramps, and charlie horses in the water, and since yesterday my left leg's ankle hurts... I don't know what to do, I want to get better at swimming but I don't know how? Feel free to share any tips/advice to get through an 1 hour of swimming at school.